Known in the art is an internal combustion engine arranging inside an engine exhaust passage an NOx storage catalyst storing NOx contained in the exhaust gas and releasing the stored NOx when feeding a reducing agent, arranging inside the engine exhaust passage downstream of the NOx storage catalyst an NO2 producing catalyst, and arranging inside the engine exhaust passage downstream of the NO2 producing catalyst an NOx selective reducing catalyst (see Japanese Patent Publication (A) No. 2006-512529). In this internal combustion engine, ammonia NH3 generated from NOx contained inside the exhaust gas is stored inside the NOx selective reducing catalyst, and NOx not stored inside the NOx storage catalyst is reduced by the ammonia NH3 at the NOx selective reducing catalyst.
In this regard, it is known that, with an NOx selective reducing catalyst, NOx is optimally reduced when the ratio between the NO and the NO2 contained in the exhaust gas is 1:1. However, the majority of NOx contained in the exhaust gas is NO. Accordingly, in this internal combustion engine, the NO2 producing catalyst is arranged upstream of the NOx selective reducing catalyst so that this NO2 producing catalyst converts the NO contained in the exhaust gas to NO2 as much as possible.
In this regard, it has been found that when using an oxidation catalyst as an NO2 producing catalyst, if the exhaust gas contains SOx, the SOx will obstruct the action of conversion of NO to NO2 and as a result lower the purification rate of NOx. However, in the above internal combustion engine, there is no suggestion of the drop in the NOx purification rate due to SOx.